[Sbcl-devel] [patch] build improvements

The attached patch should apply cleanly to any revision near 1.0.37.29.
- give make.sh a real command-line parser
- add help to make.sh
- add an option to specify the default SBCL_HOME
It was lightly tested; --prefix works for me, but I didn't fully excercise
--xc-host or "make.sh HOST".
Later,
Daniel